This invention relates in general to testing apparatus for electronic devices. More specifically it relates to a contactor assembly that is generally frequency insensitive to allow broad band testing of integrated circuits with fast-rising signals and has a connection and mounting system that is reliable and easy to use.
In the manufacture and use of integrated circuits (IC's) and similar electronic devices it is important to test the devices accurately, reliably and at a high rate. Automatic testing and handling apparatus machines that can perform this task are available. Such apparatus suitable for testing dual-in-line packaged (DIP) IC's are sold by the Daymarc Corporation, Waltham, Mass., under the trade designation Types 1157 and 757. In a DIP device, the circuit is contained in a molded plastic body having a generally rectangular, box-like configuration. Two rows of generally parallel connecting pins are arrayed along parallel sides of the body with each pin extending in a direction generally normal to the main faces of the body.
In each of the aforementioned apparatuses the IC's are momentarily brought to rest at a test station where a set of contacts, typically double Kelvin contacts, are flexed by a push bar action into electrical connection with the pins of the device under test (DUT). The contacts establish an electrical connection between testing circuitry and the device. The contacts are usually part of a probe or contactor assembly which includes an insulating base member that mounts the contacts. The contacts are typically narrow strips of a resilient and highly conductive material. The contacts typically make electrical connection with an associated connecting pin at a free end opposite the base. The cross-sectional dimensions of the contacts are relatively small due to (1) the requirement that all of the contacts simultaneously make connection with a set of closely packed pins and (2) the requirement that the contacts flex for millions of cycles of operation without material fatigue. The length of the contacts is determined by the spacing between the test station of the IC handling apparatus and the test circuitry.
Frequently the testing of the integrated circuits requires that the testing signal be "fast-rising", that is, a signal which is a very steep, step-like increase in potential. A typical fast-rising signal is characterized by a voltage change of 5 volts per nanosecond. Such a signal can be represented through Fourier analysis as being composed of a multitude of superimposed sine waves having a very high frequency, typically on the order of 300 MHz. The fast-rising signal launched by the test circuitry and carried by the contacts to the device therefore contains components with very high frequencies.
A major problem with this testing arrangement is that due to the inherent inductance of the contacts themselves, the signal encounters an inductive reactance X.sub.L. This reactance produces distortions and reflections which degrade the quality and accuracy of the test. The inductance L of the contact is a function of the cross-sectional area of the conductor and its length. Inductance increases directly with the length and inversely with the cross-sectional area. Since the inductive reactance X.sub.L =2.pi.fL, for the very high frequencies f associated with a fast-rising signal, the inductive reactance associated with even the relatively short contacts in normal use becomes a significant source of distortion and limits the accuracy of measurements.
One possible solution would be to increase the cross-sectional area of the contacts. However, the physical constraints of the test environment limit the useful dimensions of the contacts. For example, the contacts must be separated laterally from adjacent contacts while still maintaining a unique association with one pin on the IC. Also, the contacts must be sufficiently thin to flex repeatedly without exhibiting fatigue. Another possible solution is to make the contacts shorter. This solution works well if the IC can be placed manually into the test circuit. However, with high speed automated operation (e.g. 6,000 units per hour), the test circuitry must be physically separated from the device handling mechanisms with electrical connection made over some short distance spanned by a probe or contactor assembly of the type described above. In short, modern production economics require contacts having a length which is troublesome for fast-rising signals. Another possible solution is simply to test each device more slowly to wait for distortions and reflections to die out. With many modern IC's, however, the speed of operation of the device itself is so fast that if the testing operation were to extend over a sufficient period of time to allow distortions and echoes induced by the fast-rising testing signal to subside, then the speed rating of the devices cannot be determined. In short, the testing operation must have a speed comparable to that of the device function being tested.
Another consideration is minimizing "ground noise", that is, changes in the reference voltage due to current surges during the test procedure simulating operation of the device. A typical situation is a test where a change in the device state causes a current surge in the range of 20 milliamperes per nanosecond. Such a surge can cause the ground reference to move 1 volt or more thereby distorting measurements referenced to ground by 20% or more. The end result is that good devices may not pass the test and are downgraded.
Another problem with testing apparatuses for electronic devices exists in the way the probe or contactor assemblies are connected to a test circuit. When there is a high density of electrical contacts in a confined area, it is difficult to make connections with the testing circuitry while maintaining signal fidelity. The use of connectors typically introduces discontinuities which introduce reflections.
The interface assembly taught by Daymarc Corporation's U.S. Pat. No. 4,473,798 is one type of connection that avoids this disadvantage. It provides the necessary high density, characteristic impedance multiple electrical connections to multiple contacts and ground plates. It uses interchangeable contactor assemblies replaceably mounted on contactor boards. A pattern of conductive stripes is carried on at least one face of the board. Elastomeric connectors with narrow, mutually spaced apart conductive filaments electrically connect the stripes with the contacts and plates of the contactor assembly. The contactor board is clamped to the test contactor assembly at its rear surface to establish a unique electrical connection between each conductive stripe and an associated contact or plate.
While this interface assembly offers a much improved flexibility over prior connectors, under factory operating conditions there has been some difficulty in maintaining the conductive stripes of the elastomeric connectors aligned with the conductive stripes of the contactor board. It is also significant that the contactor assembly is assembled to the contactor board from the rear side and then mounted on a handler. The test circuit is then brought up and connected to the contactor board. With this arrangement, after the electrical connections through the board and the contactor assembly are tested, signal lines must be broken and re-established in order to operate the test system with a test/handler.
A further problem with the contactor assembly described in the aforementioned Daymarc '798 patent as well as U.S. Pat. No. 4,419,626 is that while a surge capacitor is connected across two pins in parallel with the device being tested to provide power-ground decoupling for fast-rising current surges, this location is far enough removed from the device that significant ground noise remains. Also, in certain test situations it is desirable to be able to connect certain pins to one another during the test so that they are at the same potential and otherwise are in the same electrical condition. With existing contactor assemblies, there has been no convenient way to short out two or more DUT pins over a short signal path that will not itself introduce reflections and distortions, particularly where the test signal has very high frequency components.
In U.S. Pat. Nos. 4,419,626 and 4,473,798, assigned to the assignee of the present invention, a contactor assembly is disclosed which is capable of testing electronic devices including high-speed ICs, without presenting significant inductive reactance to a fast-rising signal launched in any contact of the assembly. This test contactor assembly includes at least one row of flexible contacts which are secured at one end to an insulated base. A conductive plate, also secured to the base, extends to a generally parallel, closely spaced relationship to each row of contacts. The dimensions of the plate and its spacing from the associated contacts produce a distributed capacitance with respect to each contact in the row so that a fast-rising test signal launched in a contact encounters a purely resistive or "characteristic" impedance that is frequency independent.
While the contactor assembly of U.S. Pat. No. 4,419,626 gives excellent electrical performance, it does not solve the problem of reflections and distortions produced at the connection between the contact and a pin of the IC, nor does it isolate the signal on one contact from electrical distrubances produced by changes in the electrical state of the DUT. The test signal is transmitted to the device over the contacts which have a preselected "characteristic" impedence, typically in the range of 50 to 100 ohms. If the device is a 50 ohm device and the characteristic impedence of the contacts is also 50 ohms, then the transition from the contactor assembly to the device is smooth. If, on the other hand, the device has a high impedance, then the transition from a 50 ohm contactor assembly to the device results in signal reflections and signal oscillation. With this system, the device under test cannot be decoupled adequately from the test fixture and the quality of the signal seen by the device becomes uncertain. This reduces the reliability of the test.
It is also quite important to note that ideally a test fully simulates the electronic and physical environment that the DUT is likely to encounter when it is eventually used as a component of a circuit. Existing test systems have not been able to fully duplicate actual use conditions, in part because they have not been able to produce connections in a high speed test environment where a DUT pin is connected to ground over a very short signal path, as is often the case in an actual circuit, which may include capacitive or resistive circuit elements. Another shortcoming of existing contactor assemblies is that characteristic or matched impedance signal lines usually terminate in an impedance mismatch, not the characteristic impedance. It is desirable to be able to terminate a signal line in the characteristic impedance (e.g. 50 ohms) to substantially eliminate signal reflections. It is also important, if one wishes to simulate a variety of end-use circuit environments, to be able to vary the electronic test characteristics at selected DUT pins, that is, to configure and reconfigure the test environment readily and in the field. Prior art contactor assemblies do not provide this capability while also meeting the other desired operating characteristics enumerated above.
It is therefore a principal object of the present invention to provide an improved characteristic impedance contactor assembly for testing electronic devices.
Another significant object is to provide an improved contactor that decouples, or terminates in the characteristic impedance, individual pins of the device under test (DUT) through an electrical path length which is equivalent to the distance recommended for a final operative circuit for the device.
Another object of the invention is to provide this decoupling or termination in the characteristic impedance selectively at a pin or pins of the device under test.
Another object is to provide power-ground decoupling that is solder connected, low impedance and close to the device.
Another object of the invention is to provide a contactor assembly that electrically connects to a contactor or DUT board with a high degree of reliability and convenience and eliminates connectors or soldering.
Yet another object of the invention is to provide a contactor assembly that surface mounts to a contactor or DUT board from its front side adjacent the device under test so that testing of the contactor assembly to board connections does not require that signal lines be broken and then re-established to begin production testing of devices.
Another object is to provide a contactor assembly where a power-ground plane can serve as a shorting bar between two or more pins.
A further object is to provide a contactor assembly that can be field configured, or reconfigured, to the testing requirements of a device under test and which can be field maintained.
Another object of the invention is to provide a contactor assembly with the foregoing advantages that has a generally simple, low cost, and highly durable construction.